I was going to meetups and LRUG, but in most of them I just found people that talk and don't do, so I decided I stopped going until I roll out something or at least do some customer dev.
Thing is with this approach I won't meet any fellow entrepreneur, too extreme, isn't it? ;)
Having said all this, your best bet is to network and seek out introductions. You can't come in "cold", but "lukewarm" is okay, as long as there's a personal connection of some kind to your mentor -- whether it's a shared vision or a shared connection.
ETA: clarity.
If he needs mentoring on founding his startup or building a working version of his product, he's falling at the very first hurdle (beyond "having an idea"), and also the part of the process which is incredibly well documented online and elsewhere. It's formulaic.
We have two excellent NEDs at my company who are about as good as it gets, mentor-wise. But I'd never have gotten anywhere with them without first doing something.
If you want to chat sometime, I'm on Gtalk & Jabber 24/7. My email address is in my profile.
When you're doing interesting things you will meet people who are motivated to help you.
People won't take much of an interest in a person who is looking for a mentor but not doing anything.
Stop overthinking this. Establish relationships with other entrepreneurs near you. You can shoot the shit with them when things are good and when things are bad. You'll get introduced to other interesting people.
One day maybe you'll meet the fairy godmother you seem to be seeking, but if you don't you'll have made a start and know some interesting people. And, crucially, you'll be doing something.
Edit: Just saw that you're based in London. Go to the meetups and hackathons and python dojos and LRUG etc.